For some strange reason the unofficial instructions isc and dcp have ',y' addressing modes even though inc and dec do not. You can use isc and dcp here if you don't mind being a 6502 bad boy and breaking compat on some emulators.

The read-modify-write instructions (ASL, LSR, ROL, ROR, INC, DEC) have dd,X and aaaa,X modes but not aaaa,Y, (dd),Y, or (dd,X). If you're not planning to reuse the code on any 65C02 or 65816 platform (Lynx, TG16, or Super NES), you could try the unofficial RMW+ALU instructions, which have all the modes that STA has.

If you were using ca65, you'd use .setcpu "6502X" before any RMW+ALU instructions. For ASM6, you may have to edit the assembler's source code and recompile it to enable support. Or make macros in your assembly language source code that emit byte statements.

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